See gallery: 鈥�Space superlatives: The universe鈥檚 extreme performers鈥�
At the modest temperatures and pressures of Earth鈥檚 surface, the densest known material is the metallic element osmium, which packs 22 grams into 1 cubic centimetre, or more than 100 grams into a teaspoonful. Even osmium is full of fluff, however, in the form of electron clouds that separate the dense atomic nuclei. Although rarefied, these clouds are robust, and even the immense pressures deep within the planet can only compress solid matter to a modest degree.
Far greater pressure is found within the collapsed core of a giant star, a remnant we know as a neutron star. There, matter is in some exotic and ultra-dense form 鈥� most probably neutrons, and possibly a few protons and electrons, packed cheek-by-jowl. One cubic metre of 鈥渘eutronium鈥� matter from the centre of a neutron star could have a mass of up to 1018 kilograms, or a million billion tonnes.
Advertisement
An even denser hypothetical material may yet exist in the cores of neutron stars: quark matter, in which protons and neutrons dissolve into their constituent particles. The latest evidence is against it, though. Two newly discovered neutron stars are so heavy that they would probably squeeze a quark-matter core into oblivion. The clues to what really lies at the heart of a neutron star may come through studying starquakes, the juddering explosions of energy that happen when the crust of a neutron star ruptures.
Neutronium, or perhaps quark matter, may be the densest form of matter in the cosmos, but it is probably not what the densest object is made of. Compress a neutron star even further, and it will turn into a black hole. Not that all black holes are particularly dense: in fact the big ones, as measured by their event horizons, are quite tenuous. A supermassive black hole in the nearby galaxy M87 has a mass 6.4 billion times that of our sun but a density of only 0.37 kilograms per cubic metre, making it lighter than air. On the other hand, the smallest known black hole 鈥� a minnow called XTE J1650-500 鈥� is only 3.8 times the mass of the sun, but its density is just over 1018 kilograms per cubic metre. Find one of these warps in space-time that is just a little smaller, and it will overtake neutronium in the density stakes.
Microscopic black holes might also have been forged during the big bang, when quantum fluctuations in a hugely dense universe could have led to regions so dense that they collapsed. Such micro-holes might yet reveal themselves in sudden bursts of radiation: if so, this could give us an insight into the scale of quantum fluctuations in the nascent universe, and perhaps what processes actually drove the big bang.
Inside a black hole鈥檚 event horizon things get even stranger. The theory of relativity tells us that all that mass is squeezed down to a mathematical point of infinite density 鈥� though the theory almost certainly breaks down at such extremes as quantum effects begin to scramble space-time. Here, where gravity meets the quantum world, is the great frontier of fundamental physics. It is by considering such extremes as black-hole singularities that theoreticians hope to understand the most profound basis of reality.
Does a black hole鈥檚 heart conceal a fuzz-ball of wobbling strings? Or a space-sucking quantum wormhole? We don鈥檛 know, although back-of-the-envelope calculations suggest an upper limit on its density of 5 脳 1096 kilograms per cubic metre, called the Planck density. The densest thing in the universe can probably be no denser than that 鈥� whatever it actually is.
Read more: 鈥�Extreme universe: Eight cosmic record-breakers鈥�
